Acquired immune deficiency syndrome (AIDS) is a fatal disease, reported cases of which have increased dramatically within the past several years. Estimates of reported cases in the very near future also continue to rise dramatically. Consequently, there is a great need to develop drugs and vaccines to combat AIDS.
The AIDS virus was first identified in 1983. It has been known by several names and acronyms. It is the third known T-lymphocyte virus (HTLV-III), and it has the capacity to replicate within cells of the immune system, causing profound cell destruction. The AIDS virus is a retrovirus, a virus that uses reverse transcriptase during replication. This particular retrovirus is also known as lymphadenopathy-associated virus (LAV), AIDS-related virus (ARV) and, most recently, as human immunodeficiency virus (HIV). Two distinct families of HIV have been described to date, namely HIV-1 and HIV-2. The acronym HIV will be used herein to refer to HIV viruses generically.
Specifically, HIV is known to exert a profound cytopathic effect on the CD4+ helper/inducer T-cells, thereby severely compromising the immune system. HIV infection also results in neurological deterioration and, ultimately, in the death of the infected individual.
The field of viral chemotherapeutics has developed in response to the need for agents effective against retroviruses, in particular HIV. There are many ways in which an agent can exhibit anti-retroviral activity. For example, HIV requires at least four viral proteins for replication: reverse transcriptase (RT), protease (PR), transactivator protein (TAT), and regulator of virion-protein expression (REV). Accordingly, viral replication theoretically could be inhibited through inhibition of any one or all of the proteins involved in viral replication.
The PR processes polyprotein precursors into viral structural proteins and replicative enzymes. This processing is essential for the assembly and maturation of fully infectious virions. Accordingly, the design of PR inhibitors is an important therapeutic goal in the treatment of AIDS.
Anti-retroviral agents, such as 3'-azido-2',3'-dideoxythymidine (AZT), 2',3'-dideoxycytidine (ddC), and 2',3'-dideoxyinosine (ddI) are known to inhibit RT. There also exist antiviral agents that inhibit TAT.
Nucleoside derivatives, such as AZT, are the only clinically active agents that are currently available for antiviral therapy. Although very useful, the utility of AZT and related compounds is limited by toxicity and insufficient therapeutic indices for fully adequate therapy.
Numerous classes of potent peptidic inhibitors of PR have been designed using the natural cleavage site of the precursor polyproteins as a starting point. These inhibitors typically are peptide substrate analogs in which the scissile P.sub.1 --P.sub.1' amide bond has been replaced by a nonhydrolyzable isostere with tetrahedral geometry (Moore et al., Perspect. Drug Dis. Design, 1, 85 (1993); Tomasselli et al., Int. J. Chem. Biotechnology, 6 (1991); Huff, J. Med. Chem., 34, 2305 (1991); Norbeck et al., Ann. Reports Med. Chem., 26, 141 (1991); Meek, J. Enzyme Inhibition, 6, 65 (1992)). Although these inhibitors are effective in preventing the retroviral PR from functioning, the inhibitors suffer from some distinct disadvantages. Generally, peptidomimetics often make poor drugs due to their potential adverse pharmacological properties, i.e., poor oral absorption, poor stability and rapid metabolism (Plattner et al., Drug Discovery Technologies, Clark et al., eds., Ellish Horwood, Chichester, England (1990)). Furthermore, since the active site of the PR is hindered, i.e., has reduced accessibility as compared to the remainder of the PR, the ability of the inhibitors to access and bind in the active site of the PR is impaired, and those that do bind are generally poorly water-soluble, causing distinct problems in drug delivery.
The design of HIV-1 protease inhibitors based on the transition state mimetic concept has led to the generation of a variety of peptide derivatives highly active against viral replication in vitro (Erickson et al., Science; 249, 527-533 (1990); Kramer et al., Science, 231, 1580-1584 (1986); McQuade et al., Science, 247, 454-456 (1990); Meek et al., Nature (London), 343, 90-92 (1990); Roberts et al., Science, 248, 358-361 (1990)). These active agents contain a non-hydrolyzable, dipeptide isostere such as hydroxyethylene (McQuade et al., supra; Meek et al., Nature (London), 343, 90-92 (1990); Vacca et al., J. Med. Chem., 34, 1225-1228 (1991)) or hydroxyethylamine (Rich et al., J. Med. Chem., 33, 1285-1288 (1990); Roberts et al., Science, 248, 358-361 (1990)) as an active moiety which mimics the putative transition state of the aspartic protease-catalyzed reaction. Twofold (C.sub.2) symmetric inhibitors of HIV protease represent another class of potent HIV protease inhibitors which were created by Erickson et al. on the basis of the three-dimensional symmetry of the enzyme active site (Erickson et al., supra). A-77003 and other compounds designed on the C.sub.2 symmetry are undergoing clinical trials in humans (Kempf et al., Antimicrob. Agents Chemother., 35, 2209 (1991); Kempf et al., U.S. Pat. No. 5,142,056). However, peptidic compounds, such as those described by Kempf et al., which contain valinyl subunits, could undergo racemization to inactive enantiomers, i.e., enantiomers which do not demonstrate antiretroviral activity, and would, therefore, be expected to be of limited utility.
Recent studies, however, have revealed the emergence of mutant strains of HIV in which the protease is resistant to the C.sub.2 symmetric inhibitors (Otto et al., PNAS USA, 90, 7543 (1993); Ho et al., J. Virology, 68, 2016-2020 (1994); Kaplan et al., PNAS USA, 91, 5597-5601 (1994)). In one study, the most abundant mutation found in response to A7703 was Arg to Gln at position 8 (R8Q), which strongly affects the S.sub.3 /S.sub.3' subsite of the protease binding domain. Shortening the P.sub.3 /P.sub.3', residues of A-77003 results in inhibitors that are equipotent towards both wild-type and R8Q mutant proteases (Majer et al., 13th American Peptide Symposium, Edmonton, Canada (1993)). Inhibitors have been truncated to P.sub.2 /P.sub.2' without significant loss of activity (Lyle et al., J. Med. Chem., 3, 1230 (1991); Bone et al., J. Am. Chem. Soc., 113, 9382 (1991)). These results suggest that inhibitors can be truncated and yet maintain the crucial interactions necessary for strong binding. The benefits of such an approach include the elimination of two or more peptide bonds, the reduction of molecular weight, the diminishment of the potential for recognition by degradative enzymes, and the improvement of activity against certain drug-resistant strains.
The use of HIV protease inhibitors in combination with agents that have different antiretroviral mechanisms (e.g., AZT, ddI and ddT) also has been described. For example, synergism against HIV-1 has been observed between certain C.sub.2 symmetric HIV inhibitors and AZT (Kageyama et al., Antimicrob. Agents Chemother., 36, 926-933 (1992)).
The usefulness of currently available HIV protease inhibitors in the treatment of AIDS has been limited by relatively short plasma half-life, poor oral bioavailability, and the technical difficulty of scale-up synthesis (Meek et al., J. Enzyme Inhibition, 6, 65-98 (1992)). There remains an urgent need, therefore, for retroviral protease inhibitors that do not suffer from the disadvantages of currently available retroviral protease inhibitors as well as effective methods of treating retroviral infection, in particular HIV infection, involving the administration of novel antiretroviral agents alone and in combination with other antiretroviral therapies.
Accordingly, it is an object of the present invention to provide antiretroviral compounds, specifically retroviral protease inhibitors, that are resistant to viral and mammalian protease degradation and which, therefore, have improved plasma half-life and oral bioavailability. It is a related object of the present invention to provide a method of treating retroviral, specifically HIV and more specifically HIV-1 and HIV-2, infection in a mammal, specifically a human, involving the administration of one or more of the antiretroviral compounds of the present invention alone or in combination with one or more other, currently available, antiretroviral therapies. Accordingly, it is also an object of the present invention to provide pharmaceutical compositions comprising the antiretroviral compounds. Another object of the present invention is to provide a method of using such compounds to assay new compounds for antiretroviral activity and a method of synthesizing the present inventive asymmetric antiretroviral compounds so as to enable scale-up synthesis. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.